1. Related Applications
This application is related to U.S. Pat. No. 4,533,910 entitled "Graphics Display System With Viewports Of Arbitrary Location And Content", which is assigned to Cadtrak Corporation, the assignee of the present application. The disclosure of U.S. Pat. No. 4,533,910 is incorporated herein by reference.
2. Field of the Invention
The present invention relates to a computer graphics display system in which individual viewports or images are produced on a video screen with arbitrary overlapping arrangement.
3. Description of the Prior Art
In the above-identified U.S. Pat. No. 4,533,910 there is a disclosed a system for producing a graphics display on a video screen containing viewports of arbitrary arrangement, number, size and content. An objective of the present invention is to provide an improvement of such system facilitating the production of a graphics display having overlapping viewports of arbitrary arrangement and graphics content.
In the above-identified graphics display system of U.S. Pat. No. 4,533,910 production of the graphics image is accomplished utilizing a control table comprising a set of control word sequences each consisting of one or more control words. Each sequence specifies the portion of a stored graphics image which is to be displayed in a corresponding segment of a viewport associated with that control word sequence. Appropriate controller circuitry accesses from a graphics image source memory the portion of pixel data specified by each control word sequence, and displays the accessed pixel data in accordance with certain display parameters (e.g., color, zoom factor, etc.) which may also be specified by the associated control word sequence.
A further objective of the present invention is to facilitate the production of graphic displays, utilizing such a control table system, but providing for arbitrary overlapping viewports through the utilization of one or more additional control words in each control word sequence associated with an overlapping viewport display.
A significant benefit achieved by utilizing such a system is that displays containing overlapping viewports can be generated rapidly, without the necessity for transferring large blocks of graphics data each time that a change in display content or viewport arrangemnt occurs. This time saving is particularly significant when the overlapping viewport is to be displayed only temporarily, and after its removal the hidden portions of the original viewport are to be reestablished.
In prior art systems, such situation required the following operations. Initially, when say a single viewport was being displayed, a digital representation of the entire display, including all pixel data representing the single viewport, was stored in a graphics image memory. The video display was produced by raster readout from that memory.
When the display was to be changed to include a second viewport overlapping a portion of the original viewport, the pixel data representing the hidden portion first had to be moved out of the graphics display memory into some other temporary storage location. The graphics image data representing the new, overlapping viewport then had to be written into the image memory. Thus the operation required two block transfer ("BIT-BLT") operations, namely, a removal of the "hidden" original image and an entry of the overlapping viewport data. Then when the overlapping viewport was removed, the "hidden" image portion of the original viewport had to be moved back into the graphics display memory by another bit-blt operation. Where large areas of the original hidden image were overlapped, such removal and replacement bit-blt operations were time consuming and demanded significant CPU overhead.
A further object of the present invention is to provide a graphics display system facilitating the introduction and removal of overlapping viewports of arbitrary size and location, without the need for bit-blt block transfers of the hidden image data at the times when the overlapping viewport is introduced and removed.